For the inductive charging of electric vehicles, the vehicle secondary coil is preferably located on the underside of the vehicle, so that the inductive coupling with the primary coil of a charging station can be brought about by the simple parking of the vehicle on the charging station with a suitable alignment of the secondary coil with a primary coil that is located on the ground. As low as possible a design height of the secondary coil is hereby desirable, since only an installation space of low height is available for its placement on the underside of the vehicle as a rule. The secondary coil must be connected to a vehicle power electronic unit by a feedline, with the unit converting the signal coming from the secondary coil into a form suitable for the charging of the vehicle battery. The feedline must also be placed at least partially on the underside of the vehicle and is therefore subject to the same requirements as the secondary coil, with regard to the design height.
The space available in a vehicle for the placement of the additional components, such as an inductive transmission device, is generally quite limited. Therefore, a low design height of the secondary coil and its feedline is also of interest with an installation site other than the underside, such as in the area of a bumper or a crumple zone.
As a result of the relatively high operating frequency of the inductive transmission distance, which usually lies in the order of magnitude of 20 kHz nowadays and, in the future, could also shift further upward, the skin effect must be taken into consideration in the design of the feedline; it places special demands on the feedline cable. One possible solution is the use of a high frequency litz wire, which consists of a large number of very thin individual wires, insulated from one another by a lacquer layer. This type of cable, however, is relatively expensive, especially in a shielded design, and upon connection of its ends to other components, that is, here to the secondary coil and the power electronic unit, is difficult to work with.
When using a normal cable, a large conductor cross section must be selected because of the large current and the skin effect; this results in a correspondingly large total cross section of the cable. Therefore, the requirement of a feedline with a low design height, especially a design height that is not larger than that of the secondary coil, can hardly be fulfilled with a normal cable.
With a device for the inductive charging of an electric vehicle, a low design height is also desirable for the primary coil and its feedline to the charging station, in particular if the primary coil and its feedline are not lowered to the ground but rather are mounted lying on the surface of a vehicle parking space and thus are potential tripping hazards for the occupants of the vehicle, or if the primary coil is to be designed as a portable unit.
US 2001/0002773 A1 discloses a feedline cable with several conductors, insulated from one another, for the primary coil of a device for the inductive charging of the battery of an electric vehicle. The conductors are insulated from one another so as to conduct, within one single cable with an overall circular cross section, the primary current in both directions between a charging station and a primary coil, designed so it can be carried, so that only one single cable is needed to connect the charging station to the primary coil.
U.S. Pat. No. 6,649,842 B1 shows the connection of an outlet with an electrical energy source by two multi-conductor cables whose conductors can be connected on the ends without welding to the energy source or the outlet. Furthermore, this document teaches the use of ferrite cores for the compensation of the current strengths between the conductors of a multi-conductor cable. To this end, the individual conductors are joined in several stages to form groups of increasing size, and are conducted in each stage, in groups together, through ferrite cores.
U.S. Pat. No. 6,506,971 B1 shows a multi-conductor electric cable in which at least one conductor consists of several partial conductors that are insulated from one another and are connected parallel to one another, and each partial conductor is adjacent to a conductor or a partial conductor that conducts a current flowing in a phase-staggered manner or in the opposite direction, so that the magnetic field outside the cable, which is caused for the most part by the currents flowing in the cable, has as low as possible a field strength.